Slave or Proletarian?

A Proletariat according to Marx, is a "class of society which lives exclusively by its labour and not on the profit from any kind of capital." Some would consider this the definition of a slave, for a slave works and gets no money just a way to survive. Yet, Marx explains how a Proletarian and a slave are different. "The slave is sold once and for all, the proletarian has to sell himself by the day and by the hour." He also goes on to explain how "the slave is the property of one master" and "the proletarian is, so to speak, the slave of the entire bourgeois class". The slave has a guaranteed job while the proletarian is not guaranteed anything.

P.S. I commented on Abbie George's post.